This invention relates to the combustion of coal and particularly to an apparatus suitable for the partial combustion, or gasification, of coal, hereinafter referred to as a "combustor/gasifier".
With the growing concern about the availability of oil and natural gas supplies, increasing attention has been paid to the use of coal, and particularly to the use of coal in retrofit application where coal is substituted for oil or gas as a fuel in existing facilities. An attractive technology for such retrofit applications is the two-stage combustion of coal. In the first stage, external to a conventional boiler or furnace, coal is partially combusted in a fuel rich environment to produce a hot combustible gas. In the second stage, this gas is fully combusted. Since this second stage combustion may be easily carried out in the fireboxes of existing oil or gas fired facilities, the potential suitability of two-stage combustion techniques for retrofit applications is readily apparent to those skilled in the combustion art.
Large scale introduction of this technology depends on the development of a suitable first-stage combustor/gasifier. Such a combustor/gasifier would have to meet several criteria. First, the gas produced by the combustor/gasifier must be substantially free of ash when it is introduced into the second stage. Second, loss of the sensible heat produced by the partial combustion in the combustor/gasifier should be minimized in order to achieve a high over-all combustion efficiency. Third, the first-stage partial combustion and ash removal must be carried out at high flow rates to achieve low cost and reliable close-coupling with the second stage. Further, the capital and operating costs of the combustor/gasifier must be economical.
Previous external combustors or gasifiers have typically been of the cyclone, slagging type where a fuel stream of pulverized coal or a coal slurry and combustion air are introduced into the combustor with a swirling motion and the ash is thrown to the walls by the induced centrifical force and removed as molten ash or "slag". This swirl-mixing gives an intense, stable combustion which allows a compact structure.
Such cyclone type combustors operate at a temperature above the fusion point of the coal (i.e. at a temperature such that the ash is liquid). The liquid ash, or slag, is thrown to the walls of the combustor which are maintained at a temperature above the fusion point of the slag so that the slag remains liquid and thus may be drained off through a "slag tap".
This type of slagging operation has, however, several disadvantages. The slag causes severe corrosion problems as well as problems of slag build-up and fouling. The fouling problems are aggrevated by water in the coal which tends to cause a need for expensive coal drying facilities and to preclude the use of coal/water slurries.
A second problem is that the high temperature required at the walls for the slagging operation are dangerously near to the useful temperature limits for available cost-effective wall materials. This problem is compounded by the fact that currently economical coal cleaning techniques tend to raise the fusion point of coal ash. Further, coal with an appreciable ash content must be used since an appreciable amount of ash is needed to effectively control the wall temperature.
It is also known to use fixed or fluidized bed combustors as combustor/gasifiers. Such combustor/gasifiers generally effectively remove the ash, but have an unsatisfactory low through-put for a given size and poor reliability due to slag tap plugging.
Thus, it is an object of the subject invention to provide a cyclone-type combustor/gasifier which produces a dry ash.
It is a further object of the subject invention to provide a combustor/gasifier having integral capabilities for ash removal.
It is another object of the subject invention to provide a compact, economical, combustor/gasifier capable of close-coupled high through-put operation.
It is still another object of the subject invention to provide a combustor/gasifier having a minimal loss of sensible heat to the environment while at the same time operating at combustion temperatures low enough for dry ash operation.